This is a resubmission of a competing renewal of RO1 DA08425, Serotonin and Aggression in Cocaine Dependence. The title of renewal has been changed to reflect the newly defined emphasis on the impact of impulsivity on the treatment of cocaine dependence. Cocaine Dependence is associated with higher levels of impulsivity, both clinically and as measured by questionnaires and laboratory measures. We have found that impulsivity is associated with a poor outcome in the treatment of cocaine dependence. Further, there is evidence from our group and others that impulsivity is associated with decreased functioning of the neurotransmitter serotonin, and that serotonergic medication increases response to contingency management treatment for cocaine dependence. In light of the above data, we propose a treatment study to determine whether the serotonin reuptake inhibitor citalopram improves response to contingency management treatment of cocaine dependence. We also propose to measure serotonin and impulsivity to determine whether baseline serotonin function, and changes in impulsivity predict response to treatment. The Specific Aims of this proposal are: Aim 1. To determine whether chronic administration of the 5-HT re-uptake inhibitor citalopram improves response to contingency management treatment of cocaine dependence. Aim 2. To measure pretreatment Serotonin (5-HT) function using a citalopram neuroendocrine challenge and determine the relationship between 5-HT function and response to treatment in cocaine dependent subjects. Aim 3. To measure impulsivity in subjects undergoing treatment for cocaine dependence using laboratory tasks to determine if changes in impulsivity predict response to treatment for cocaine dependence.